As known in the art, commercially available silicas can broadly be divided into two basic categories. These categories are those produced from a liquid phase and those from a vapor phase process.
Vapor process silicas, called fumed and pyrogenic silicas, are prepared by reacting silicon tetrachloride vapor with an oxygen-hydrogen gas at high temperatures to produce silicon hydrogen chloride. Pyrogenic silicas have high external surface areas and differ from other silicas (e.g., gels, precipitated silicas, etc.) prepared from the liquid phase process.
Liquid phase silicas include precipitated silicas produced by acidulating an alkaline metal silicate with an acid such as sulfuric acid. Further liquid phase silicas include silica gels and colloidal silicas.
While precipitated silicas have been used in many applications, various properties thereof (such as high oil absorption values) render them unsuitable for many purposes.
In recent years, and to further expand the utility of precipitated silicas, prior art workers have developed new techniques for producing silica having new and unique properties.
Thus, there is disclosed in U.S. Pat. No. 2,731,326 a process for producing reinforced super colloidal silica aggregates consisting of extremely small, dense, amorphous silica units which are dried to produce a product which can be readily disintegrated and dispersed in a fluid or plastic medium. According to the process, aggregates of dense silica spheroidal units are reinforced by causing active silica to accrete to the aggregates. The accreted silica acts as a cement or mortar to hold spheroids together. These silica aggregates have both high surface areas and high oil absorption characteristics.
U.S. Pat. No. 3,967,563 discloses a process for precipitating a very high structure silica. In the disclosed process, the rate of acidulation is very carefully controlled, particularly at the outset of the reaction and at the point at which precipitation initially occurs. The silica product produced by this method has a very high structure, a wet cake moisture between 86-89%, an oil absorption of about 220-250 cc/100 g and a surface area of about 250-280 m.sup.2 /g as shown by examples 1-4.
U.S. Pat. No. 4,251,281 discloses a process for producing synthetic amorphous silica which has a surface area of between 80-310 m.sup.2 /g and an oil absorption index of between 240-320 cc/100 g. The process involves reacting a strong acid with an alkaline silicate solution wherein the acid is added to the solution at a variable rate to maintain a residual alkalinity of the medium substantially constant with time.
Thus, while techniques have been developed for producing amorphous precipitated silicas having various improved properties, improvements directed to such silica which have high wet cake moisture contents and reduced oil absorption values have not been made.
Inasmuch as reduced oil absorption values have the added value of lowering volatile organic compounds (VOC) requirements in coating formulations versus other precipitated silica flatting agents, there is a commercial need for silica products which have reduced oil absorption values and high wet cake moisture values. This is particularly true since present VOC regulations are in effect in certain areas of California and nationwide regulations are expected in the near future. Thus, paint and coating manufacturers are keenly interested in a low oil absorption quality for silica products that can be used as flatting pigments.